A relationship between in vitro platelet responsiveness and atherogenesis is shown in foxhound dogs and Hormel miniature swine. In the presence of significant hyperlipidemia (Chol greater than 1200; tgy greater than 600, both ADP and collagen induced platelet aggregation are affected. A general relationship between disease severity and platelet sensitivity is being further investigated. After months on experimental diet, platelet survival decreases (only) in animals rendered atherosclerotic. Significant changes in platelet fatty acid composition (see Pitas, et al, Lipids, in press) are associated with functional changes. Dose-response platelet aggregavity is necessary to detect the functional change.